This application is based on Japanese Patent Application No. 2000-133634 filed on May 2, 2000, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a multi-stage heating furnace arranged to heat a stack of large-sized substrates in the form of glass plates used for a liquid crystal display (LCD) panel and a plasma display panel (PDP), and a heating type far-infrared-radiation panel heater. The present invention is also concerned with a method of controlling flows of a fluid (air) through the heating furnace.
2. Discussion of Related Art
In the manufacture of large-sized substrates for the liquid crystal display (LCD) panel and plasma display panel (PDP) indicated above, it is indispensable to perform drying and heating steps for drying and heating the substrates after washing and after application of a coating, to remove a washing liquid and a solvent contained in the coatings. To this end, a heating furnace is required. The substrates are thin rectangular sheets each side of which has a length of about 1.0-2.0 m and the thickness of which is about 0.7-3.0 mm. Thus, the substrates to be dried and heated are considerably large.
Heating furnaces generally used to dry and heat such large-sized substrates include, for instance: (1) a heating furnace of hot-air circulating type; (2) a heating furnace of far-infrared-radiation type; and (3) a heating furnace of hot-plate type. The heating furnace of hot-air circulating type can be easily designed to heat the substrates arranged in a stack, and has an advantage in the efficiency of space utilization. However, the heating furnace of hot-air circulating type suffers from difficult control of temperature distribution and contamination of the substrate with a dust blown by the hot air. Therefore, this type of heating furnace is not suitable for drying the large-sized substrates, where the substrates are required to be dried in a clean atmosphere with an even distribution of the heating temperature over the entire area of the substrate surface.
The heating furnace of far-infrared-radiation type has the following advantages: improved heating efficiency for removing the solvent; a comparatively high degree of accuracy of control of the heating temperature for even distribution over the entire range of the substrate surface, by controlling a heater; and a comparatively high degree of cleanliness of the atmosphere within the furnace, owing to radiation heating without circulation of a hot air. However, the heating furnace of far-infrared-radiation type is generally required to position its heating portion (adapted to generate a far infrared radiation) in opposed relationship with the substrates, for effecting the radiation heating of the substrates. Accordingly, the heating furnace of this type is usually required to be a tunnel furnace (continuous furnace) in which the substrates to be dried are fed one after another along a horizontal path by a suitable conveying mechanism. The tunnel type heating furnace must have a relatively large overall length and accordingly requires a relatively large installation space, where the substrates are required to be dried to a comparatively large extent per unit time or where the substrates have a relatively large size, as indicated above.
The heating furnace of hot-plate type is arranged to effect conduction heating on the lower side of the substrates, and therefore has advantages of even or uniform heating (even distribution of the heating temperature) and a relatively high rate of rise of the heating temperature, where the substrates have a relatively small size. Where the substrates are relatively large, however, this type of heating furnace suffers from the following drawbacks: difficulty to achieve even heating: lower heating efficiency for removing the solvent contained in the coating applied to the upper surface of each substrate, than in the heating furnace of far-infrared-radiation heating; and requirement for a relatively large installation space where the hot-plate type heating furnace is designed to be a tunnel or continuous type furnace.
It is therefore a first object of the present invention to provide a multi-stage heating furnace which is capable of heating large-sized substrates in a clean atmosphere with a high degree of accuracy of temperature control and which requires a reduced installation space.
A second object of the invention is to provide a far-infrared-radiation panel heater suitable for use in the heating furnace of the present invention. A third object of the invention is to provide a method suitable for controlling flows of a fluid through the heating furnace of the invention.
The first object indicated may be achieved according to a first aspect of this invention, which provides a multi-stage heating furnace for drying a plurality of large-sized substrates arranged in a stack, the multi-stage heating furnace comprising:
a furnace body; and
a plurality of shelf heaters arranged within the furnace body such that the shelf heaters are spaced from each other in a vertical direction at a predetermined spacing pitch and such that adjacent ones of the shelf heaters partially define a plurality of drying chambers for accommodating the plurality of large-sized substrates, respectively, each of said plurality of shelf heaters consisting of a far-infrared-radiation panel heater of double-side heating type including a heat radiating plate in which a heat-generating body is embedded, the panel heater having opposite major surfaces covered with respective thin ceramic layers which emit a far infrared radiation when the heat radiating plate is heated by energization of the heat-generating body.
In the multi-phase heating furnace constructed according to the first aspect of this invention, each of the shelf heaters arranged within the furnace body in vertically spaced-apart relation with each other includes a heat radiating plate incorporating a heat-generating body, and has opposite major surfaces covered with respective thin ceramic layers for emitting a far infrared radiation, so that heat is radiated on the opposite sides of each panel heater. The electric wires and terminals for energizing the heat-generating body may be accommodated in the peripheral portion of the panel heater.
The shelf heaters consisting of the respective panel heaters thus constructed are spaced apart from each other within the furnace body, so as to partially define the drying chambers. In operation of the heating furnace, the large-sized substrates are placed on the respective shelf heaters, via suitable support blocks, so that each substrate is heated concurrently on its opposite sides, by radiation heating by the adjacent shelf heaters, whereby the substrates are suitably dried. Owing to the arrangement of the shelf heaters consisting of the double-sided far-infrared-radiation panel heaters in the vertically spaced-apart relation with each other within the furnace body, the space required for installing the present multi-stage heating furnace is significantly reduced with respect to that of a conventional tunnel type heating furnace, for drying a given number of large-sized substrates in one drying cycle. Further, each panel heater of the shelf heaters can be made comparatively thin, so that the shelf heaters can be arranged at a relatively small spacing interval or pitch in the vertical direction. Accordingly, the required height of the present multi-stage heating furnace can be reduced.
The present multi-stage heating furnace is capable of heating the large-sized substrates for liquid crystal display (LCD) panels and plasma display panels (PDP), in an atmosphere having a high degree of cleanliness, with a high degree of accuracy of control of the heating temperature, so that the quality of the dried large-sized substrates can be significantly improved. This advantage in the operating performance of the present heating furnace is enjoyed in addition to the above-indicated advantage of the reduced installation space for the heating furnace owing to the multi-stage shelf heaters arranged in the vertical direction within the furnace body.
According to a first preferred form of the first aspect of the invention, the far-infrared-radiation panel heater further includes a cover plate disposed above the heat radiating plate, and the outer surface of the heat radiating plate and the outer surface of the cover plate are covered with the thin ceramic layers for emitting the far infrared radiation.
According to a second preferred form of the first aspect of the invention, the heat-generating body embedded in the heat radiating plate consists of a plurality of heating portions located in respective different temperature control zones which are arranged in a horizontal direction in which the large-sized substrates are introduced into and removed from the furnace body, the plurality of heating portions being arranged to heat corresponding portions of the heat radiating plates to respective different temperatures that are determined to establish a substantially even distribution of a temperature over a substantially entire surface area of each of the large-sized substrates.
In the heating furnace according to the second preferred form of the invention described above, the temperature at each large-sized substrate can be substantially evenly distributed over the entire surface area of the substrate, owing to the different heating temperatures in the local portions of the heat radiating plate which corresponds to the respective different temperature control zones of the heat-generating body embedded in the heat radiating plate extending in the horizontal direction. The temperature control zones are arranged in the horizontal direction in which the substrates are introduced into and removed from the furnace body.
The heating furnace according to a third preferred form of the invention further comprises a pair of side heaters which are disposed as auxiliary heaters on opposite sides of a stack of the shelf heaters, so as to extend in the vertical direction along opposite side walls of the furnace body, each of the side heaters consisting of a far-infrared-radiation panel heater.
In the heating furnace according to the third preferred form of the invention described above, the accuracy of control of the temperature at the large-sized substrates can be further improved owing to the side heaters which extend vertically along the respective side walls of the furnace body, so as to heat the peripheral portion of each large-sized substrate in the corresponding drying chamber. The heating furnace may further comprise a rear heater disposed vertically along a rear wall of the furnace body.
The heating furnace according to a fourth preferred form of the invention further comprises at least one bottom heater consisting of a far-infrared-radiation heater which cooperates with a lowest one of the shelf heaters to define at least one bottom dummy drying chamber, and at least one top heater consisting of a far-infrared-radiation heater which cooperates with an uppermost one of the shelf heaters to define at least one top dummy drying chamber, none of the large-sized substrates being accommodated in the bottom and top dummy drying chambers in operation of the multi-stage heating furnace.
In the heating furnace according to the fourth preferred form of the invention described above, the temperatures in the lowermost and uppermost drying chambers can be controlled to be equal or close to those in the vertically intermediate drying chambers, in the presence of the bottom and top dummy drying chambers, which function to compensate for a difference between the temperatures in the lowermost and uppermost drying chambers and the temperatures in the vertically intermediate drying chambers, so that the temperatures at the large-sized substrates to be dried in the lowermost and uppermost drying chambers can be controlled with a higher degree of accuracy.
The second object indicated above may be achieved according to a second aspect of this invention, which provides a far-infrared-radiation panel heater of double-side heating type used in a multi-stage heating furnace for drying a plurality of large-sized substrates arranged in a stack within a furnace body, the far-infrared-radiation panel heater comprising:
a heat radiating plate in which a heat-generating body is embedded; and
two thin ceramic layers which covers opposite major surfaces of the panel heater and emit a far infrared radiation when the heat radiating plate is heated by energization of the heat-generating body.
According to a first preferred form of the second aspect of this invention, the far-infrared-radiation panel heater of double-side heating type further comprises a cover plate disposed above the heat radiating plate, the two thin ceramic layers covers an outer surface of the heat radiating plate and an outer surface of the cover plate, respectively.
According to a second preferred form of the second aspect of the invention, the far-infrared-radiation panel heater of double-side heating type further comprises a hollow frame structure in which the heat radiating plate is fixedly fitted over an entire periphery thereof, the heat radiating plate partially defining a fluid passage communicating with an interior of the hollow frame structure, the heat radiating plate having a multiplicity of blow holes communicating with the fluid passage and open in an outer surface thereof, for fluid communication between the fluid passage and a space outside the panel heater.
The third object indicated above may be achieved according to a third aspect of this invention, which provides a method of controlling flows of a fluid through a multi-stage heating furnace constructed according to the first aspect of this invention, the method comprising the steps of:
blowing a fluid downwards from a lower surface of each of the plurality of shelf heaters, into an internal space within the furnace body, through blow holes open in the above-indicated lower surface;
discharging the fluid from the space into an external space outside the furnace body, by suction through suction holes formed for fluid communication between the internal and external spaces through the furnace body.
In the fluid control method according to the third aspect of this invention described above, the fluid such as air is blown downwards from the lower surface of each shelf heater into the internal space of the furnace body, through the blow holes formed through the shelf heater, and is discharged into the external space outside the furnace body, by suction through the suction holes formed through the furnace body. According to this arrangement, the fluid is pre-heated by the shelf heater before it is blown into the internal space the furnace body, so that the temperature distribution at the upper surface of each substrate is not undesirably influenced by the blown fluid. Accordingly, solvents evaporated from the substrate under drying, and the heat generated within the furnace body are discharged or dissipated into the external space, whereby the atmosphere within the furnace body is kept considerably clean, and the substrate is effectively dried by the flows of the fluid.
According to a preferred form of the third aspect of the invention, the above-indicated step of discharging the fluid comprises forming the suction holes through a pair of side heaters disposed on respective inner surfaces of opposite side walls of the furnace body. In this arrangement, the fluid blown into the internal space can be effectively discharged into the external space through the suction holes formed through the side heaters, under the aid of suction through the suction holes connected to a suitable sucking device such as an exhaust fan.